The energy consumed by electric motors used in industry comprises a high proportion of overall electrical energy consumption. A major cause of excessive energy consumption is the improper sizing of motors to match the load to be driven. Consumption of electricity can therefore be significantly reduced by replacing improperly sized motors with motors which are properly matched to the load. In order to determine where this may be beneficial in a particular case, it is necessary to accurately evaluate the efficiency and loading of an existing motor and thus determine the maximum power required by the process.
Previously there have been four methods typically used to determine the efficiency of an electric motor:
(1) Equivalent circuit test methods--the performance of an electric motor can be calculated from its equivalent electric circuit if all elements in the circuit have been determined from experimental test data. Tests are performed under no-load and load conditions in order to obtain sufficient data for the solution of equivalent circuit network equations in an iterative procedure. Assumed impedances are modified until calculated values of power and power factor agree with measured values.
There is a high probability of error which limits the reliability of this method.
2) Segregated loss methods--this method is based on the premise that motor efficiency is equal to the input power minus losses, divided by the input power. Accurate measurement or estimation of power losses at no-load and full-load conditions can be difficult, particularly in older motors which have suffered some deterioration, and inaccuracies will affect the efficiency of the measurement obtained.
3) Dynamometer measurement--this test method requires the use of a calibrated power-absorbing dynamometer capable of handling the full load of the motor. A torque transducer introduced between the motor and the dynamometer, or a torque arm and load cell, is used for measurement of the reaction torque delivered to the dynamometer. Efficiency is determined directly from the measured output power and the electrical input power. Since the entire output of the motor must be absorbed by the dynamometer, this method is most useful for evaluating motors under 200 horsepower. Furthermore, the motor must be decoupled from its load and coupled to the dynamometer to make this measurement, resulting in considerable expense and down time.
4) Strain gauge torquemeters--in this device a strain gauge is bonded to or embedded in a shaft. Signals from the strain gauge are transmitted to a measuring apparatus using slip rings bonded to the shaft and electrically coupled to stationary brushes. The motor is decoupled from the load, and the torquemeter is coupled between the motor and the load. The signals transmitted by the strain gauge under load are compared to calibrated values and converted to a measurement of torque. To determine efficiency, the motor shaft speed and input power are also measured, and related to the torque measurement as described below. While this method has proved accurate and reliable, considerable expense and down time is encountered in decoupling the motor from the load and introducing the torquemeter.
The present invention provides a strain gauge torquemeter for determining the efficiency and loading of a motor under load conditions, without decoupling the motor from the load. A strain gauge is affixed directly to the motor drive shaft, and signals from the strain gauge are transmitted through a telemetry data coupler requiring no direct connection to the drive shaft. These signals provide an accurate value for torque, which, when multiplied by the motor speed, provides an accurate measurement of the output power of the motor. The motor efficiency can then be determined by dividing the output power by the input power as measured by a power transducer.